Historically, telecommunications have involved the transmission of voice and fax signals over a network dedicated to telecommunications, such as the Public Switched Telephone Network (PSTN) or a Private Branch Exchange (PBX). Similarly, data communications between computers have also historically been transmitted on a dedicated data network, such as a local area network (LAN) or a wide area network (WAN). Currently, telecommunications and data transmissions are being merged into an integrated communication network using technologies such as Voice over Internet Protocol (VoIP). Since many LANs and WANs transmit computer data using Internet Protocol (IP), VoIP uses this existing technology to transmit voice and fax signals by converting these signals into digital data and encapsulating the data for transmission over an IP network. However, the integration of telecommunications and data transmissions is still ongoing, and many features that were available to users of traditional telecommunications networks have not been made available to users of VoIP and similar technologies.
The reliability of PBXs and central offices (COs) has improved over the years and is said to meet the high mark of five nines. To address the same requirements for the VoIP Market, substantial effort has gone into handling failures in call managers (CMs) and the attached servers. Both PBXs and call managers rely on a backup processor that takes over call processing tasks in the case where the primary call manager fails. With the fail over mechanism of the call manager, users whose phones are connected to a call manager that fails can continue their call but can not invoke any supplementary services. After the calls are terminated, the phones that were homed to the failed call manager re-home to the backup call manager and provide full feature service to the users.